Lighting, and more specifically high intensity discharge (HID) lighting, makes up a large portion of the energy costs at many industrial, commercial, and public facilities. In many cases, full light output is not required during certain hours of the day or under specific operating conditions. Therefore, in order to achieve costs savings, it would be beneficial to control this lighting source in order to provide substantial energy cost reductions.
Currently, methods and apparatus exist to reduce the applied voltage to an HID load in a stepwise manner using solid state switching, and transformer tap switching. These methods usually have the drawback of being too costly and/or too complex.
One such prior art attempt is shown in FIG. 1 which comprises apparatus 10 for switching an output terminal 12 from a first voltage to a second voltage using an autotransformer 14, a first switch 16, a second switch 18, a third switch 20, and a resistive element 22. The resistive element 22 acts as a transitional buffer to facilitate the switching of the voltage at the output terminal 12 from an input voltage source 24 to the output of the autotransformer 14. For the system to provide full voltage, the first switch 16 is initially closed. In order to change from the first voltage (full voltage) to the second voltage, a transition is made whereby the third switch 20 is closed and the first switch 16 opened. The second switch 18 is then closed and the third switch 20 re-opened. As can be seen, the switches 16, 18 and 20 are located on the secondary side of the autotransformer 14 which requires the presence of the resistive element 22 to prevent damage to the switches 16, 18 and/or 20. This prior apparatus has several drawbacks such as the fact that the third switch 20 is required to facilitate the transition between the first and second voltages. Furthermore, the resistive element 22 is generally a large wattage restive element and is a required element of the apparatus 10. Also, the switches 16, 18 and 20 are in series with the output terminal 14 and must therefore be rated for the full load current being supplied by the input voltage source being controlled. Moreover, this method requires four switching operations (and therefore a more complex timing sequence) to transition from the first voltage level to the second voltage level.
It is, therefore, desirable to provide a novel apparatus and method for controlling voltage in lighting systems and more specifically in high intensity discharge lighting loads.